[ntuple] Fix item type names for STL map fields

[enirolf]

With the addition of RIntegralTypeMap (PR https://github.com/root-project/root/pull/16039), most of the type name deduction for type-erased fields was moved from the type translation map to RField's template specialization. Upon creation of type-erased STL map(-like) types, originally only the type translation map was used. This now can cause issues where the map's item types may be non-normalized. By using the type names of the subfields of the std::pair item field, we ensure the correct inner type names are used.